This application claims priority to British Patent Application No. 0025247.8, filed Oct. 13, 2000.
The invention relates to an improved method of optical proximity correction, and products thereof.
As the resolution limit of an optical lithography tool is approached, the fidelity of printed features is compromised by the size and location of their neighbours resulting in reduced dimensional control. These xe2x80x9coptical proximityxe2x80x9d effects include dense-isolated bias, ie, effects resulting from changing linewidth density.
In photolithography lines are defined by passing light through a xe2x80x9creticlexe2x80x9d which acts as a mask and is typically formed from glass printed with chrome patterns. In order to print lines the reticle is provided with lines and spaces which allow the light to pass through onto the photoresist (a photosensitive layer which covers the substrate which is to be etched using photolithography) and these lines can be of varying pitch. For repeating lines the pitch refers to the spacing of the lines, thus the greater the pitch the more isolated the lines. If the wavelength of the light approaches the size of the lines, their thickness (linewidth) can be affected. It is possible to correct for such variations in the printed linewidth on the wafer by changing the linewidths on the reticle. Such corrections are made automatically using optical proximity correction (OPC) software packages, of which there are now several commercially available brands, resulting in selective modification of the linewidths in the reticle design to achieve the desired printed image.
The applicant, Mitel Semiconductor Limited, has already developed a method of applying OPC through lithography simulation using the correction software CAPROX OPC (RTM) in conjunction with the optical lithography simulation tool SOLID-C (RTM), thereby allowing the entire procedure to be carried out without recourse to practical experiment or having restrictions imposed by the limitations of aerial image only correction (described in a paper by Arthur, G., Martin, B., Wallace, C. and Rosenbusch, A. entitled xe2x80x9cFull-Chip Optical Proximity Correction using Lithography Simulationxe2x80x9d presented at BACUS Photomask Symposium in September 1998.). A flow diagram for using CAPROX (RTM) is shown in FIG. 1.
OPC is normally applied at exposure-to-size for dense lines (ie, those having equal line/space ratio) and lines at other pitches are corrected using an appropriate linewidth versus pitch function. Alternatively the exposure-to-size for isolated lines could be used and lines at other pitches corrected. A description of OPC itself is given for example in Wallace, C., Duncan, C. and Martin, B. xe2x80x9cApplication of Optical Proximity Correction Manufacturing and its Effect on Process Controlxe2x80x9d Metrology, Inspection and Process Control for MicrolithographyXlV, SPIE, 2000.
According to the invention there is provided a method of optical proximity correction, and reticles and polysilicon gates produced by such a method, as set out in the accompanying claims.
The use of a selected exposure corresponding to a relatively flat contour of the dense-isolated offset focus/exposure matrix is desirable because it results in a manufacturing process having greater resilience to surface topography of the wafer.